1. Field of the Invention
The present invention relates to a process for the production of stable curdled milk products through the inoculation of a raw milk material with bacteria cultures.
2. Discussion of the Prior Art
Until recent times, during the production of stable curdled milk products, a raw milk material such as, for instance, such as whole milk or skim milk, is inoculated with mixed cultures which, as a rule, contain strains belonging to two different types of bacteria. Heretofore, yoghurt-like, stable curdled milk products with a pleasant aroma have been obtained when the mixed culture, in addition to bacilli characteristic for the product, of the type Lactobacillus bulgaricus, also contained bacteria of the type Streptococcus thermophilus. Lactobacillus bulgaricus strains curdle milk extensively up to a pH of 3.65 and are responsible for the typical yoghurt aroma. However, this type of bacteria is disadvantageous in that it produces the undesirable D(-) lactic acid (levorotatory) and does not deliver a stable product but merely a loose coagulate. In contrast therewith, the second type of bacteria present in the mixed culture, S. thermophilus curdles the milk only relatively weakly up to a pH of 4.8 and does not deliver a typical yoghurt aroma. Contrastingly, this type of bacteria produces the desired L(+) lactic acid (dextrorotatory) and, as a result, is responsible for the curdled milk product becoming stable. Inasmuch as neither of the two types of bacteria which are employed in a typical yoghurt culture in themselves possess the required metabolic properties which are essential to the sought-after aromatic and stable curdled milk product, both types must be employed conjointly. However, the disadvantages of both must also be concurrently taken into consideration. The following table illustrates the advantageous and disadvantageous properties of the bacteria strains which are utilized in the mixed cultures pursuant to the present state of the art.
______________________________________ Bacteria type Advantages Disadvantages ______________________________________ L. bulgaricus aroma formation, rapid formation of curdling bitter components, curdling, formation of D(-) lactic acid, no stable product S. thermophilus stable product, no after slow curdling, curdling, formation of no aroma L(+) lactic acid ______________________________________
Since recently doubts have been expressed in an increasing measure from a medical standpoint about the use of levorotatory lactic acid, the World Health Organization (WHO) recommends that the daily intake of D(-) lactic acid be restricted to 100 mg per kilogram of body weight. Inasmuch as stable curdled milk products, such as yoghurt, are consumed in large amounts, it appears to be of great importance to extensively reduce the quantity of D(-) lactic acid in such products. However, until now it has not been possible to produce stable curdled milk products with rod bacillus-like lactobacilli which do not contain D(-) lactic acid.
In the utilization of strains of bacteria, for instance of the Lactobacillus bifidus type, which produce only the desired dextrorotatory lactic acid, until the present time there could only be produced only liquid curdled milk products. Accordingly, it has been attempted to produce stable curdled milk products through the use of mixed cultures which also contain strains producing only dextrorotatory lactic acid. Nevertheless, these were displaced within a short time span by strains of bacteria present in the culture which produce the levorotatory lactic acid. For example, when a raw milk product is inoculated with a mixed culture which contains the same proportion of levorotatory lactic acid producing lactobacilli and the dextrorotatory lactic acid producing strain of the bacteria, Bifido bacterium longum, within a few days the latter is displaced by the ratio of 1000:1.
A further problem which is encountered in the utilization of bacteria cultures which do not contain mixtures of the different types of material, but only one or more strains of a single type of bacteria, is the danger of attack by phages. Since, as a rule, phages evidence only a narrow selective effectiveness, in essence, they will mostly infect bacteria of one type: in actual practice one can protect from attack by phages by using mixtures which contain strains of different types of bacteria. In the case of an attack by phages, at a minimum there is eliminated, only one type of bacteria so that the ripening or digesting process will not be entirely disrupted. Consequently, in the current state of technology it has not been considered feasible to employ so-called monocultures with the utilization of only one type of bacteria as the inoculating material for stable curdled milk products.